This invention relates to heat-insulating paper containers, and more particularly to heat-insulating paper cups used for coffee and other hot or cold beverages.
Several types of heat-insulating cups have been used commercially for hot and cold liquids. Examples include the wide variety of existing polystyrene foam cups. These cups are typically made by adding a foaming agent to a polystyrene resin, casting the mixture into a mold, heating the resin under pressure to foam it, and removing the shaped article from the mold. Such cups have outstanding heat-insulating properties, but require a lot of energy to make, and are costly to dispose in an environmentally acceptable way.
A variety of paper-based cups have been proposed as environmentally acceptable alternatives to polystyrene containers. Unfortunately, most attempts to produce paper-based heat-insulating cups have proved costly due to the complexity of their manufacturing processes. One example is a cup where the side wall is surrounded by a corrugated heat-insulating jacket. Its process for manufacture involves additional steps of forming the corrugated paper jacket and bonding it to the outer surface of the side wall of the cup. These cups, however, have proved to be aesthetically unappealing and structurally deficient. In this regard, only the ridges of the corrugated jacket contact the body of the paper cup in such a way that the bond between the two is so weak that the cup and jacket easily separate. Also such cups are not easily nested, making storage difficult.
Another type of insulating paper cup is one in which the cup has a dual cup structure. An inner cup is given a different taper than an outer cup to form a heat-insulting air layer in between the two cups. The two cups are integrated by curling their respective upper edges into a brim. The dual structure adds to manufacturing costs, and the two cups of the structure are prone to separation.
One of the most widely accepted types of heat-insulating paper-based cups include those described in U.S. Pat. No. 4,435,344, and also referred to in U.S. Pat. No. 5,490,631. Those cups have good insulting properties and can be prepared at a relatively low cost. Such cups are fabricated from a body member and a bottom member, both cut from a paper sheet.
For the cups described in the aforementioned U.S. patents, one surface of the body member is coated or laminated with a thermoplastic synthetic resin film, and the other surface of the body member is coated or laminated with the same or different thermoplastic synthetic resin film or an aluminum foil, to thereby foam the thermoplastic synthetic resin film and form a heat-insulting layer on at least one surface of the container, i.e., the outer surface. Water present in the paper is vaporized upon heating during processing, causing the thermoplastic resin film on the outer surface to foam.
Commercial versions of cups prepared according to U.S. Pat. No. 4,435,344 include a body member and a bottom panel member. The body member comprises a paper sheet coated or laminated on one side with a foamed heat-insulating layer of low density polyethylene which entirely covers its outer surface. The inner surface of the body member is covered by with an unfoamed film of high density polyethylene. The bottom panel member is laminated on its inner or upper surface with low density polyethylene film. The inside surface of the body member, and the inside surface of the bottom member are coated to prevent penetration of liquid contents into the paper sheet during use. The inside surface of the body member is also coated to ensure that the water in the paper will not evaporate directly into the air atmosphere during heating of the fabricated cup.
For these commercial cups, the film to be laminated on the inner surface of the body member is high density polyethylene because it has a higher softening point than the low density polyethylene film on the outer surface. Upon heating during fabrication, the moisture inherently present in paper sheet serves as the foaming agent for the outer surface layer of low density polyethylene film. The high density polyethylene on the inside surface of the body member will not foam under fabrication conditions but serves to seal the interior of the cup thus preventing an escape of the moisture necessary to foam the low density polyethylene on the outer surface of the cup.
An improvement over the technique of U.S. Pat. No. 4,435,344 is described in copending U.S. application Ser. No. 08/870,486, filed Jun. 6, 1997, the contents of which is incorporated herein by reference. U.S. application Ser. No. 08/870,486 describes, among other improvements, an improved seal between the body member and bottom member.
Commercial cups are usually printed with a variety of logos, designs, and other printed matter. In this regard, for flexographic and rotogravure printing on polymer substrates, water-based binder resins formed from acrylic acid and its derivatives have been used in the inks of choice. When they are applied to the substrate, and dried, they rapidly form tough, durable resin films which are highly desirable, particularly with respect to beverage containers where the printed matter requires high levels of physical and chemical abuse resistance.
The inventors have found a problem with paper-based cups like those prepared according to U.S. Pat. No. 4,435,344. That problem relates to the application of printed text such as logos, etc., to their surfaces. Commercially available modified acrylic water-based inks printed on the resin surface inhibit insulation formation, with the amount of inhibition dependent on ink layer thickness. This characteristic can be beneficial in the case where an embossed effect is desired. See co-pending U.S. application Ser. No. 08/604,783, filed Feb. 23, 1996, the contents of which is hereby incorporated by reference. However, that same characteristic can also be detrimental when maximum insulation is needed in the same area as the printed matter. For example, it is frequently desirable with hot beverage cups to position a printed logo in the very same area where protective insulation is needed the most, i.e., that place where the cup is held by the user's hand. While full print coverage. i.e., 100% coverage, often will not lower the insulative effect below acceptable levels (e.g., that level of a conventional double cup), the inhibition of insulation formation does impose undesirable limitations such as the exclusion of overprints (double layers of ink).
It was, therefore, an object of the invention to provide a cup like that prepared according to the techniques of U.S. Pat. No. 4,435,344, and U.S. application Ser. No. 08/870,486, which did not exhibit the aforementioned undesirable limitations on insulation formation associated with cups printed using typical acrylic-based inks.